The gonadotropin-releasing hormone (GnRH) neurons of the hypothalamus form the final site of integration for central signals regulating reproductive function. It has been suggested that within this population of cells, functional subsets exist which serve different physiological roles. For example, the release of GnRH from these GnRH-containing neurons in the female exhibits two distinct modes: during the follicular phase, GnRH release is strictly episodic. During the preovulatory GnRH surge, however, GnRH release appears to be comprised of a high frequency episodic release and a non-episodic component. The non-episodic mode of GnRH release may reflect recruitment of a surge-specific population of GnRH neurons. In this regard, anatomical studies have suggested that structural plasticity at the level of the GnRH nerve terminal may result in the "recruitment" of a previously non-neuroendocrine population of GnRH neurons. Using differential labeling, the present proposal will examine electrophysiological and morphological properties of neuroendocrine GnRH neurons and those GnRH neurons putatively recruited to neuroendocrine status at the time of the preovulatory surge. Specific Aim 1 will test the hypothesis GnRH neurons recruited to neuroendocrine status at the time are distributed through the rostral hypothalamus and express c-fos during the GnRH surge. Specific Aim 2 will test the hypothesis that GnRH neurons recruited at the time of the surge express similar morphology but temporally distinct patterns of action potential firing (i.e., slow basal firing vs. burst firing). These preliminary studies are intended to begin to explore the general hypothesis that functional differences exist within the GnRH neuronal population in a paradigm likely to reveal such differences if they exist.